chore: switch to all-SQLite stack (drop Postgres service)

backend/app/db.py

@@ -1,6 +1,7 @@
 """Database engine, sessions, and FastAPI dependency."""
 
 from collections.abc import Iterator
+from pathlib import Path
 
 from sqlalchemy import Engine
 from sqlmodel import Session, SQLModel, create_engine
@@ -10,11 +11,23 @@ from .settings import settings
 _engine: Engine | None = None
 
 
+def _engine_kwargs(url: str) -> dict:
+    if url.startswith("sqlite"):
+        # FastAPI runs sync endpoints in a threadpool; connections are shared
+        # across threads, so defer the same-thread check. File-based SQLite
+        # needs an existing parent dir (SQLite won't create it).
+        return {"connect_args": {"check_same_thread": False}}
+    return {}
+
+
 def get_engine(url: str | None = None) -> Engine:
     """Process-wide engine singleton (override URL in tests via settings)."""
     global _engine
     if _engine is None:
-        _engine = create_engine(url or settings.database_url)
+        url = url or settings.database_url
+        if url.startswith("sqlite:///") and not url.startswith("sqlite:///:memory:"):
+            Path(url.removeprefix("sqlite:///")).parent.mkdir(parents=True, exist_ok=True)
+        _engine = create_engine(url, **_engine_kwargs(url))
     return _engine